A Fabry-Perot interferometer comprises a first semi-transparent mirror and a second semi-transparent mirror, which are arranged to form an optical cavity. The Fabry-Perot interferometer may provide one or more transmission peaks. The spectral position of the transmission peaks may be changed by changing the distance between the mirrors. The distance between the mirrors may be called as the mirror gap or as the mirror spacing.
The interferometer may comprise e.g. a piezoelectric actuator for adjusting the mirror spacing. The piezoelectric actuator may be driven by applying a driving voltage to the actuator, wherein each voltage value of the driving voltage may be associated with different mirror spacing. The value of the mirror spacing may be determined from the corresponding value of the driving voltage e.g. by using a regression function. The regression function may be determined by varying the mirror spacing, and by monitoring transmission of a laser beam through the interferometer. When the transmitted intensity reaches a maximum, the value of the mirror spacing may be calculated from the wavelength of the laser beam by using the Fabry-Perot transmission function.